Piezoelectric damper hones ski performance

Cambridge, MA--In what's called the first commercial application of "smart structures" technology, the K2 Four ski from K2 Corp., Vashon Island, WA, incorporates a piezoelectric damping module from Active Control eXperts (ACX), Inc., Cambridge, MA. The damper selectively reduces vibration to improve edge control for better turning and quicker runs.

The physics of skiing involves the reaction between ski edges and the snow. Uneven snow surfaces cause skis to vibrate, lessening the contact area of edge to ice and reducing the ability of the skier to control turning forces. "Skis need damping," says Kenneth Lazarus, president of ACX. "A lot of techniques have been tried, but the piezoelectric damper is the first that's unobtrusive and relatively unaffected by temperature."

Unlike conventional dampers, the ACX device dissipates mechanical energy as heat by first converting it to electricity then passing it through a resistive shunt. Measuring approximately 6.62 x 1.66 x 0.07 inches, it delivers better than 30% added damping. What makes it "smart"? Although a purely passive device (the piezoelectric material is not driven to counteract vibration), its shunt circuit has been tuned to damp only those vibratory modes that adversely affect ski performance.

Anthony DeRocco, director of alpine product development at K2, explains that identifying those modes required a lengthy research effort in cooperation with ACX and with dynamicists at Boeing. For example, just developing fixtures to hold the ski for vibrational testing so as to correlate with on-snow data took months. Fully a year went by to optimize the Four's design.

Nevertheless, the piezoelectric damper went through only a few iterations, says Adam Bogue, director of marketing at ACX. One attempt delivered too much damping; ski testers called it unresponsive. Later designs scaled back the damping and optimized the directional response of its components.

The reason for the relative ease of damper design comes from the thought given to ACX's smart-structure design. About half the company's patents cover the technology's manufacturing processes, which integrate the brittle piezoelectric components and electronic circuitry in a single, flexible package. The packaging reduces crack propagation in the ceramic piezoelectric material and permits any surface-mount electronic component to be included in the control circuit. Says Bogue, "It's a marriage of piezoelectrics and flex circuits."

Although the ski-damper design is passive, the smart-structures technology it's based on also has uses in sensing and applying force. To demonstrate, Bogue connects one of the company's PZT QuickPack™ devices to a variable-frequency power supply. With the power connected for bending response by the device, it begins to wiggle like a rapidly heated and cooled bimetal coupon. Held by one end, the four-inch-long device's free end traverses a full half inch. Connected for a tension-compression response and bonded to a surface, the QuickPac produces as much as 70 lbf.

ACX has several programs in the works to use the technology in active noise suppression, sensor, valve-actuator, and precision motion-control applications. Company President Lazarus explains that co-location of actuator and control greatly simplifies system design. For example, a smart-structure noise suppression system might not require a digital signal processor. "We attack the vibration at the source, not the noise it creates," he says.

Structure-borne noise control

Force and pressure sensing

High-bandwidth actuators

As for the company's work with K2, Lazarus says he's grateful for the chance to demonstrate smart structures as commercially viable in an easily understood application. And though ski-making includes other concerns--the Four ski includes what's described as a "radical" side cut--K2's DeRocco acknowledges the importance of the ACX damper. "At 198 cm, it's as stable as a 207-length ski," he says. "It's a new class of high-performance ski."

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